1. Field of the Invention
This invention relates to a lens array and a close contact type image sensor using the same. The lens array of the present invention is particularly suitable for use in a close contact type image sensor adapted to be in close contact with an original and to effect reading.
2. Related Background Art
FIG. 14 of the accompanying drawings is a cross-sectional view showing the structure of a close contact type image sensor according to the prior art. As shown in FIG. 14, a sensor unit is comprised of a sensor module (constituting a sensor array) 4 comprising a sensor IC 1 having a plurality of pixels for effecting photoelectric conversion arranged, protective film 2 and a substrate 3 having these mounted thereon, an LED array 5 which is a light source for illuminating an original, a lens element body (constituting a lens array) 6 for forming the image of the original on the light receiving portion of the sensor, a transparent member 7 in close contact with the original 9 to support it, and a frame 8 supporting these members.
As regards the operation of the above-described close contact type image sensor, the original 9 is illuminated by the light source (LED array 5), diffused reflected light on the read line of the original is imaged on the pixel array of the sensor by the lens element body 6, the light and shade information of the original had by the reflected light, i.e., the intensity of the light, is converted into an electrical signal by the individual sensor pixels and is delivered in the main scanning direction. The relative position of the original and the sensor pixel array is then moved in the sub-scanning direction and the data delivery in the main scanning direction is repeated to thereby convert two-dimensional image information into a time-serial electrical signal.
As the lens element body, use can be made, for example, of one comprising a plurality of lens elements each having a refractive index distribution in a direction orthogonal to the optical axis. More specifically, use can be made of one comprising a number of optical fibers each having a refractive index distribution in a direction orthogonal to the optical axis and arranged on a straight line, and since the individual optical fibers exhibit a lens action, the lens element body as a whole acts as a small and long imaging lens. FIG. 15 of the accompanying drawings shows the opening portions of the lens element body, and is a view of the lens element body 6 as it is seen from the sensor module 4 side. In FIG. 15, the reference character 6a designates the opening portions of the lens element body, the reference characters 6b and 6c denote support plates, and the reference character 6d designates a member of resin or the like holding the lens elements.
In the above-described example of the prior art, however, a lens element body, i.e., a double-eye lens, is used as the imaging lens and therefore, image overlap occurs and the combined angle of opening becomes great, and this has led to a problem that the depth of field is shallow and the quality of image is deteriorated when the original is bent or is uneven with cuts sticked thereon. Also, in a flat bed scanner or the like as shown in FIG. 19 of the accompanying drawings, it has been impossible to read a spread of a book or the like and the use of the sensor has been limited.
The manner of imaging will now be described with a grated index type optical fiber lens array as shown in FIGS. 16A and 16B of the accompanying drawings taken as an example of the lens element body. As shown in FIG. 16A, individual lens elements image erectly, at one to one magnification, an area of diameter Xo on the surface of an original on the surface of the sensor. A number of such lens elements are arranged on a straight line as shown in FIG. 16B, whereby the images of the individual lens elements are overlapped to thereby form a long image surface. The depth of field when an image is formed by the combination of the images of a plurality of lens elements is considered as follows. When as shown in FIGS. 17A and 17B of the accompanying drawings, a certain point (object point P) and its image q are supposed, a larger number of lens elements concerned in imaging leads to a result that the combined angle of opening .theta.' becomes greater than the angle of opening .theta. of each individual lens element and the blur B of the image q becomes greater relative to the movement .delta.L of the object point P'. That is, the depth of field of the lens element body becomes shallow, as compared with the depth of field of each individual lens element. FIG. 18 of the accompanying drawings shows the relation between the number of lens elements concerned in imaging and the depth of field. Here, a parameter which is the overlap degree m=X/D is introduced by the diameter X of the image and the diameter D of each lens element to thereby represent the number of lens elements concerned in imaging. The depth of field can be improved by making the angle of opening .theta. of each individual lens element small, but in this case, the optical path length TC (P-q) becomes great, and when such lens element body is used in a close contact type image sensor as in the example of the prior art, the body becomes bulky. Also, the quantity of light reaching the image surface decreases remarkably and the burden applied to the light source becomes great. Further, such a lens element body having a great depth of field is expensive.